/Engine/Plugins/Experimental/ChaosCaching/Source/ChaosCaching/Public/Chaos/ChaosCache.h
As we record post-simulate of physics, we're almost always taking data from a non-main thread (physics thread). Because of this we can't directly write into the cache, but instead into a pending frame queue that needs to be flushed on the main thread to write the pending data into the final storage.
void FlushPendingFrames()